1. Field of the Invention
The invention relates to a voltage conversion circuit apparatus and, more particularly, to a voltage conversion circuit apparatus that adjusts a timing skew between switching controls of at least two switching elements.
2. Description of Related Art
Recent years have seen the development of various electric power supply systems in which an electric power supply is constructed by using secondary cells, fuel cells, etc. In such electric power supply systems, for example, a fuel cell system, the output voltage of the fuel cell is converted by a DC-DC converter, and the converted voltage is supplied to a load side. In some cases, a soft-switching circuit construction is adopted in order to reduce the loss of a voltage-boosting switching element of the DC-DC converter. As for the soft-switching circuit construction, various circuits have been devised. Among those, a soft-switching circuit construction that has an auxiliary switch that actively controls a snubber circuit in addition to a main switch that controls a voltage boost circuit is able to perform soft-switching operations in a broad operation region.
An example of the soft-switching circuit construction is a circuit construction that includes a snubber capacitor element provided in parallel with a voltage-boosting switching element. The provision of the snubber capacitor element restrains the occurrence of spike voltage at the time of switching operation of the voltage-boosting switching element. However, despite the soft-switching operation having been performed in order to lessen the switching loss by using this construction, the switching loss sometimes cannot be sufficiently lessened because, in some cases, there is electric charge accumulated in the snubber capacitor element prior to the switching operations of the voltage-boosting switching elements. Therefore, there is provided a charge-removing switching element for performing a switching operation in order to remove the charge accumulated in the snubber capacitor element, prior to the switching operation of the voltage-boosting switching element.
As a technology related to the invention, Japanese Patent Application Publication No. 2009-165245 (JP-A-2009-165245) discloses a construction of a fuel cell system that includes a fuel cell as a direct-current power supply, a voltage boost portion that boosts the output voltage of the fuel cell and applies the boosted voltage to a load, and voltage boost control means for controlling the voltage boost performed by the voltage boost portion. Then, as for the voltage boost portion, this publication discloses a construction that a main voltage boost portion that increases the output voltage of the fuel cell by causing main switch means to perform a switching operation on a main coil that is connected to a high-potential-side terminal of the fuel cell, and a subsidiary voltage boost portion which has a snubber capacitor that is connected in parallel with the main switch means and that is capable of adjusting the voltage that is applied to the main switch means and in which the applied voltage of the snubber capacitor is adjusted according to the voltage boosting operation of the main voltage boost portion. Furthermore, as for the voltage boost control means, the foregoing publication also discloses a construction in which when the output voltage is to be boosted by the voltage boost portion through the switching operation of the main switch means, the voltage boost control means removes charge from the snubber capacitor before the main switch means performs a predetermined switching operation.
By the way, it is necessary that a first pulse signal for performing a switching control of the voltage-boosting switching element (first switching element) and a second pulse signal for performing a switching control of the charge-removing switching element (second switching element) be input to the corresponding switching elements in a predetermined timing relation, and that the switching controls of the two switching elements be performed in a desired timing relation. However, because of differences in the signal propagation time between the pulse signals or of process variations of the switching elements, a timing skew between the switching controls of the switching elements may occur, making it impossible to switch the switching elements in a desired timing relation. This will give rise to a possibility of failing to sufficiently remove the charge accumulated in the snubber capacitor, prior to the switching operation of the voltage-boosting switching element.